1. Field of the Invention
The present invention relates to a liquid crystal display apparatus wherein a voltage is applied in a direction parallel to the surface of a pair of substrates sandwiching a liquid crystal layer, and a manufacturing method of the liquid crystal display apparatus.
2. Description of the Related art
Conventionally, a liquid crystal display apparatus using a nematic liquid crystal displaying element is widely employed for a numerical segment type display apparatus of watches, desk calculators, etc. The light transmitting substrate composing such liquid crystal displaying apparatus is provided with an active element as switching means for selectively driving a picture element electrode which applies a voltage to a liquid crystal. In addition to that, color filter layers of red, green, and blue colors are provided as color display means. The liquid crystal displaying types are classified in accordance with the twist angles of liquid molecules into an active drive type twisted nematic (hereinafter described as TN) liquid crystal displaying mode, where nematic liquid crystal molecules are oriented with a twist angle of 90.degree., and a multiple drive super twisted nematic (hereinafter described as STN) liquid crystal displaying mode, which takes advantage of a steep relationship between light transmittances and voltages applied to the liquid crystal by twisting the nematic liquid crystal molecules by 90.degree. or more.
Since specific coloring is found in the latter multiplex drive STN type, an optical phase difference element is provided in the case where black-and-white displaying is conducted. The optical phase difference element is roughly classified into a two layers type double super twisted nematic liquid crystal displaying method and film added type liquid crystal displaying method. A liquid crystal cell for displaying and a liquid crystal cell oriented with a twist angle in a reverse direction are used in the former method. On the other hand, a film having optical anisotropy is arranged in the later method. In view of decreasing in weight and cost, the latter film added type liquid crystal displaying method is desirable.
On the other hand, the active drive-type TN liquid crystal displaying mode is roughly classified into a normally black mode, in which the polarizing directions of a pair of polarizing plates are arranged in parallel to each other and displaying is carried out in black color in a state that a voltage is not applied to a liquid crystal layer, and a normally white mode in which the polarizing direction thereof are arranged in perpendicular to each other and displaying is carried out in white color in an off-state. In view of the contrast of an image, the color reproducibility, and the dependence of an image on viewing angle, the normaly white mode is desirable.
However, the contrast of a displayed image varies depending on a viewing angle, and the dependence of the contrast on a viewing angle is high in the liquid crystal displaying apparatus employing the TN displaying mode because liquid crystal molecules have refractive anisotropy and are arranged with an inclination to the substrate. Particularly, a reversal phenomenon that black and white colors of an image is reversely observed occurs when a viewing angle formed by a viewing direction and a direction perpendicular to the image plane is beyond a certain angle.
In order to improve the dependence of the liquid crystal display apparatus on a viewing angle, the phase difference between the components of normal light and abnormal light is conventionally tried to be compensated by interposing between an element and a polarizing plate a phase difference plate (film) where the direction of one principal index of refraction is in parallel with a direction perpendicular to an image plane in the index ellipsoid. To use the phase difference plate, however, has a limitation in improving the reversal phenomenon of a black-and-white image.
Further, a liquid crystal displaying mode where a voltage is applied in parallel with a substrate surface has been proposed as another method for improving the viewing angle dependency by R. A. Sorer ( J. Appl. Phys. 45, 5466(1974)) and R. Kiefer et al. (JAPAN DISPLAY' 92 P2-30(1992)). Since a voltage is applied not in perpendicular to, but in parallel with the substrate surface in this liquid crystal display mode, the liquid crystal molecules do not incline to the substrate. Consequently, the viewing angle dependency is symmetrical in bilateral- and vertical-directions of the image plane in the case where the viewing angle is inclined from a normal line direction of the image plane.
FIG. 11 is a sectional view showing a constitution of a conventional liquid crystal display apparatus 10. The liquid crystal display apparatus 10 comprises light transmitting substrates 1, 2, transparent electrodes 3a, 3b, orientation films 4, 5, a liquid crystal layer 6, an adhesive agent 7, and polarizing plates 8, 9. The transparent electrodes 3a, 3b practiced by a material such as indium tin oxide (ITO) are formed on a one surface 1a of a substrate 1 practiced by a material such as glass. For instance, a distance of about 10 .mu.m is given as the distance d1 between the transparent electrodes 3a, 3b. A thickness d2 ranging from hundreds to thousands A is given as the thickness d2 between the transparent electrodes 3a, 3b. The orientation films 4, 5 are formed on the one surface 1a of the light transmitting substrate 1 where the transparent electrodes 3a, 3b are formed and on a one surface 2a of a transparent substrate 2, practiced by a material such as glass like the light transmitting substrate 1, respectively. The orientation films 4, 5 are practiced by a material such as a polyimide resin manufactured by JAPAN SYNTHETIC RUBBER CO. LTD., whose surface is treated by rubbing with a cloth threaded with, for instance, nylon fiber for orientation treatment. A one substrate member 11 comprises the light transmitting substrate 1, the transparent electrodes 3a, 3b and the orientation film 4, and the other substrate member 12 comprises the light transmitting substrate 2 and the orientation film 5.
One surface 1a of the light transmitting substrate 1 and one surface 2a of the light transmitting substrate 2 are arranged to face to each other, and are bonded to each other with the adhesive agent 7. A liquid crystal such as a nematic liquid crystal is injected between the substrate members 11, 12 to form the liquid crystal layer 6. For example, a thickness of about 5 .mu.m is given as an example of the thickness d3 of the liquid crystal layer 6 and a refractive anisotropy of 0.0791 is given as an example of the reflective anisotropy .DELTA.n. The orientation direction of the orientation films 4, 5 formed on the light transmitting substrates 1, 2 are determined so that liquid crystal molecules 6a of the liquid crystal layer 6 interposed between the substrate members 11, 12 are oriented to be twisted by 90.degree. between the substrate members 11, 12. The polarization plates 8, 9 are arranged on the other surfaces 1b, 2b of the light transmitting substrates 1, 2, respectively.
FIG. 12 is a view for explaining the displaying principles of the liquid crystal display apparatus 10. An electric field as shown by dashed lines of FIG. 10 is generated in the liquid crystal layer 6 by applying a voltage between the electrodes 3a, 3b of the liquid crystal display apparatus 10, and the liquid crystal molecules 6a are oriented along the electric field direction. For instance, the light incident through the polarizing plate 9 to the liquid crystal layer 6 when a voltage is not applied is twisted by 90 .degree. in a vibration direction by virtue of the liquid crystal molecules of the liquid crystal layer 6. The light outgoing from the liquid crystal layer 6 passes through the polarizing plate 8 and a white color is displayed. On the other hand, the light incident through the polarizing plate 9 to the liquid crystal layer 6, when a voltage is applied, outgoes from the liquid crystal layer 6 without being twisted in the vibration direction. The light which went out from the liquid crystal layer 6 is blocked and a black color is displayed.
The thickness d2 of the transparent electrodes 3a, 3b of the liquid crystal display apparatus 10 is very thin as compared that of the liquid crystal layer 6 arranged between the substrate members 11, 12. When a voltage is applied between such transparent electrodes 3a, 3b, the liquid crystal molecules 6a existing almost just above the electrode 3a, 3b are oriented nearly in perpendicular to the light transmitting substrates 1, 2, and the other liquid crystal molecules 6a are oriented nearly in parallel with the transparent substrates 1, 2. Consequently, the orientation of the liquid crystal molecules 6a are non-uniformly oriented, which causes non-uniform display and a decrease in image contrast. As a result, the image decreases in quality.